Past in subordinate clauses

Staying close to your original form, you could say,

"I don't want to see what he might have written" or "I don't want to see what he might be writing."

You can keep your message simpler, since you don't want to see the reference (not hypothetically), and the reference will be what he writes (not hypothetically):

"To keep it private, I don't want to see what he writes."

1. Secrecy -- A reference is useless if it's not positive. So you may assume it will be positive, no harm in that. And you will see it in the process of the translation, so don't worry anymore about secrecy.
2. Translation -- If you are thinking that the translation will look like you tampered with the content, either through your involvement or in the translation, you can add a note like "Professionally translated from the original."

For the first sentence, if he didn't actually write it, then you would say neither one. You'd say, "I wouldn't want to see what he would've written."

For the second sentence, since you're not already talking in the past tense, you wouldn't say "had taken." You'd say, "I'd take the thing, and then you'd take the thing I took." Only if you were using "would" in the first main clause to mean the past-tense of will would you use the pluperfect "had taken" (e.g., "Every day after the bell went off, I would take the thing, and then you'd take the thing that I had taken.").

For the third sentence, since you're saying it in chronological order, there's no reason to use the pluperfect tense. You'd say, "I took my keys before you came home." Now if you reversed the order of the clauses, the pluperfect tense would be proper (i.e., "Before you came home, I had taken my keys.").